1. Field of the Invention
This invention relates to a wireless door locking and unlocking system for locking and unlocking the door of a motor vehicle having a theft alarm device. Further, the wireless door locking and unlocking system may be available for a door or a window of a house or the like. The car theft alarm device detects a car theft positively to produce an alarm signal.
2. Description of the Related Art
A conventional wireless door locking and unlocking system is as shown in FIG. 1. In FIG. 1, reference numeral 1 designates an antenna; 2, an RF amplifier; 3, a local oscillator; 4, a mixer; 5, an intermediate frequency (IF) amplifier; 6, a detector; and 7, a control circuit comprising a microcomputer.
The system thus organized operates as follows: A transmitter (not shown) transmits a radio wave which is the modulation signal of an ID code and function code. The radio wave is received by the antenna 1, and then amplified by the RF amplifier 2. The radio wave thus amplified is converted into an intermediate frequency (IF) signal with the aid of the local oscillator 3 and the mixer 4. The IF signal is amplified by the IF amplifier 5. The output signal of the IF amplifier 5 is detected by the detector 6; that is, the ID code and function code are detected by demodulation of the radio wave. The output signal of the detector 6 is applied to the control circuit 7, and the latter 7 outputs a locking or unlocking signal in accordance with the codes.
In the conventional wireless door locking and unlocking system, one and the same circuit is used for locking and unlocking the door. Therefore, both in locking the door and in unlocking the door, the longest signal receivable distance from the transmitter is the same. On the other hand, sometimes the door may be at a signal receivable distance which is so long that the operator cannot detect visually whether the door has been locked or unlocked. If, in this case, the door is unlocked by mistake, then the following trouble may occur. In this case, the operator, being at the long distance from the door, cannot see the door having been unlocked, and therefore he may leave the vehicle (or house) although the door is not locked resulting in that theft may occur.
FIG. 2 is a block diagram showing another conventional wireless door locking and unlocking system. In FIG. 2, reference numeral 21 designates a door locking and unlocking switch; 22, a trunk unlocking switch; 23, a code generator; 24, a door locking and unlocking oscillator; and 25, a trunk unlocking oscillator. The door locking and unlocking switch 21, the trunk unlocking switch 22, the code generator 23, the door locking and unlocking oscillator 24, and the trunk unlocking oscillator 25 constitute a transmitter 26.
Further in FIG. 2, reference numeral 27 designates a door locking and unlocking receiver unit; 28, a trunk unlocking receiver unit; and 29, a control circuit. The door locking and unlocking receiver unit 27, the trunk unlocking receiver unit 28, and the control circuit 29 constitute a receiver 30.
The operation of the system thus organized will be described. Upon operation of the door locking and unlocking switch 21 or the trunk unlocking switch 22, a radio wave is modulated with an ID code and function code in the code generator 23. The transmitter 26 transmits the radio wave thus modulated.
The radio wave thus transmitted is received by the door locking and unlocking receiver unit 27 or the trunk unlocking receiver unit 28, and then converted into a predetermined intermediate frequency (IF) signal. The intermediate frequency signal is detected. That is, the ID code and function code are detected by demodulation of the radio wave. The resultant signal is applied to the control circuit 29, and the latter 29 outputs a door locking signal, a door unlocking signal, or a trunk unlocking signal in accordance with the codes thus detected.
It is essential for the above-described conventional wireless door locking and unlocking system to have the door locking and unlocking oscillator 24, the door locking and unlocking receiver unit 27, the trunk unlocking oscillator 25 and the trunk unlocking receiver 28. Therefore, the system is relatively high in manufacturing cost. In addition, both in locking the door and in unlocking the door, the longest signal receivable distance from the transmitter 26 is the same. On the other hand, sometimes the door to be operated may be at a signal receivable distance which is so long that the operator cannot visually detect whether the door has been locked or unlocked. Accordingly, as is similar to the above-described convention system, the vehicle (or house) may also suffer from theft.
An example of conventional car theft alarm devices which are to be equipped to motor vehicles having the wireless door locking and unlocking system is for instance as shown in FIG. 3. In FIG. 3, reference numeral 31 designates a battery on a vehicle; 32, a door lamp of the vehicle; 33, a drive switch for the door lamp 32; 34, a brake lamp; 35, a drive switch for the brake lamp 35, which switch is provided at the base of the brake pedal; and 36, an ignition key switch.
Further in FIG. 3, reference numeral 37 designates a car theft alarm device. The device 37 comprises a 60-second timer 37b with a relay contact means 37a, a reference voltage generating circuit 37c, a voltage comparison circuit 37e and an alarm drive circuit 39.
The 60-second timer 37b is started with an alarm set switch 38 to output a time-up signal after sixty (60) seconds has elapsed. The alarm set switch 38 is mounted on a part of the driver's seat which part is difficult for persons other than the driver to locate.
The reference voltage generating circuit 37c outputs a reference voltage in response to the time-up signal from the 60-second timer 37b. The reference voltage is normally set to a value which is slightly lower than the rated voltage of the vehicle battery 31.
The voltage comparison circuit 37e compares the reference voltage outputted by the reference voltage generating circuit 37c with the output voltage of the vehicle battery 31 applied to a power supply line PL so as to output an alarm instruction signal when the output voltage of the vehicle battery 31 is detected lower than the,reference voltage. The alarm instruction signal is applied to the alarm drive circuit 39.
In response to the alarm instruction signal, the alarm drive circuit 39 applies an alarm drive signal to an alarm sound generating section (not shown).
The operation of the car theft alarm device thus organized will be described with reference to a flow chart of FIG. 4.
Before leaving his vehicle with the car theft alarm device, the driver turns on the alarm set switch 38 installed on the driver's seat.
When the alarm set switch 38 is turned on in this way, the 60-second timer 57b is started, so that the relay contact 37a is closed after sixty (60) seconds have elapsed and the time-up signal is then applied to the reference voltage generating circuit 37c (Steps S100 and S101). The reference voltage generating circuit 37c generates the reference voltage corresponding to the rated voltage of the vehicle battery 31 to be supplied to the comparison circuit 37e (Step S102). The comparison circuit 37e compares the current output voltage of the vehicle battery 31 with the reference voltage. Thus, the car theft alarm device is placed in standby state (Step S103).
Thereafter, if a third person opens the door to enter the vehicle, the door lamp 32 is turned on. And if the third person operates the vehicle, the brake lamp 34 is turned on, and power is supplied to the starter, so that the output voltage of the vehicle battery 31 is decreased abruptly. When the output voltage becomes lower than the reference voltage provided by the reference voltage generating circuit 37c, the comparison circuit 37e issues the alarm instruction signal to the alarm drive circuit 39, and the alarm sound generating section sounds the alarm (Step S104).
In the conventional car theft alarm device thus organized, the reference voltage, which is compared with the present output voltage of the vehicle battery 31 in the comparison circuit 37e, is set to slightly lower than the rated voltage of the vehicle battery 31. Hence, the device suffers from the following problem. If the output voltage of the vehicle battery becomes lower than the reference voltage for instance because the vehicle battery is deteriorated, or carelessly the room lamp is not turned off, then the alarm sound generating section will produce an alarm. This is a false alarm.